1. Field of the Invention
This invention relates in general to the field of infrared (IR) image detection. More particularly, the invention relates to a quantum grid infrared photodetector having means for enhancing the direction of propagation of incident IR radiation.
2. Description of the Prior Art
A quantum well IR photodetector (QWIP) is a superlattice semiconductor device which functions to produce intersubband transitions within a conduction band when a ground state electron is promoted to an excited state upon absorbing an incoming photon having energy equal to the subband spacing. Once in the excited state, the electron freely moves within the QWIP to form a photocurrent under electrical bias. As such, QWIP's are often used to detect IR radiation. QWIP arrays have been used to detect IR images.
The physical construction of a conventional QWIP generally includes a stack of alternate semiconductor material layers sandwiched between two contact layers. The layers are grown on a transparent semiconductor substrate and cover an area that is relatively broad in comparison to the layer thicknesses. A typical semiconductor material system suitable for QWIP fabrication is GaAs/Al.sub.x Ga.sub.1-x As.
In a conventional QWIP of the type just described, intersubband optical transitions can be initiated only if a component of the electric field vector of the incident IR radiation is normal to the broad surface areas of the semiconductor layers in the stack. Consequently, IR radiation that is incident normal to the semiconductor layers cannot be absorbed by the QWIP. Because only IR radiation having components directed parallel to the semiconductor layers can be absorbed by the QWIP, attempts have been made to provide structures that can redirect the incident IR radiation closer to the desired parallel direction.
One prior art technique for redirecting IR radiation in a QWIP uses a grating coupling technique. The efficiency of a grating coupled QWIP is discussed in the following published article: Lundqvist et al., "Efficiency of grating coupled AlGaAs/GaAs quantum well infrared detectors, "Applied Physics Letters, vol. 63 (24), 13 Dec. 1993, pp 3361-3363.
In a typical grating coupled QWIP, grooves are etched into the upper contact layer and a continuous metal contact is then deposited on the upper surface to form an optical grating that diffracts non-parallel incident radiation into different discrete directions. In general, such diffracted radiation will have a finite electric field component that is perpendicular to the broad layer surfaces, causing some of the IR radiation to be absorbed. Although this diffraction grating technique for coupling IR radiation into a QWIP has been used successfully in making two-dimensional IR detector arrays, it has several drawbacks. First, in order to make an efficient diffraction grating, extra thick contact layers (e.g., 2.5 microns or greater) have to be grown, which can be costly. Also, detector quantum efficiency can be appreciably reduced because extra radiation absorption, which does not contribute to photocurrent, takes place in the highly doped contact layer which helps form the grating.
Second, plasma etching of the contact layer to form the grating often causes material damage to layers underneath the grating. The damaged regions, often having thicknesses up to 1.5 microns, may be highly conductive, causing a number of detector semiconductor layers to be shorted out. Since only the remaining undamaged layers are still active, the sensitivity of the detector is reduced.
Third, in grating coupling, the partially absorbed light can be internally refracted by a substrate-air interface into different detector pixels of a detector array. This radiation can cause appreciable crosstalk between the different detector pixels.
Fourth, the diffraction of light by a grating is effective only when the overall grating dimension is much larger than the groove spacing. However, in a high-resolution detector array, the number of grooves at each pixel is usually small, which reduces the grating efficiency. In order to avoid this problem and to enhance the absorption within the same detector pixel, the substrate can be thinned by polishing. However, to make the thinning effective, the substrate has to be thinned to only a few microns thick, making the whole detector array extremely fragile.
Fifth, because radiation having only a narrow range of wavelengths is diffracted to a proper angle for each grating periodicity, a grating coupler cannot be used to couple a wide range of radiation with different wavelengths and, hence, limits the spectral response of a QWIP. Therefore, manufactures have to design a specific grating structure for each different detector. Also for the same reason, grating coupling cannot be used effectively for multi-color detection.